1. Field of the Invention
This invention relates to oscillators of the feedback amplifier type, and more specifically to oscillators employing a bipolar complementary transistor pair for the amplifier.
2. Description of the Prior Art
RC and crystal oscillator circuits are frequently used in logic circuitry for timing and control purposes. Often the amplifiers for such oscillators are constructed from integrated circuit CMOS logic inverters or logic gates so arranged. These amplifiers are conveniently powered from the existing CMOS power supply and have the advantage of directly interfacing with CMOS logic (this logic being noted for its relatively high impedance inputs and hence low loading effects).
In FIG. 1 a crystal oscillator, noted for its stability and accuracy, is shown employing a CMOS inverter 38 and a phase-shifting feedback network 22 having a high Q piezoelectric crystal 28 (Eaton, U.S. Pat. No. 3,725,822). Resistor 34 biases the amplifier in the active region (as is also true of some RC oscillators). As long as the amplifier-feedback network loop gain exceeds unity regenerative action takes place and oscillations are sustained.
In CMOS logic the supply source appears across series connected P and N channel FET gate-to-source junctions. As FET saturation current varies as the square of the difference of gate-to-source voltage from a threshold voltage, substantial currents can result when using CMOS logic ICs, particularly as may be judged for some battery applications. Alternately, if one chooses a CMOS supply voltage near the sum of the FET threshold voltages then oscillations may not occur on power-up owing to insufficient loop gain.
Thus, in some battery-sources applications and especially where supply voltage changes occur, an undesirable high supply current may have to be accepted when using CMOS inverters or gates.